1. Field of the Invention
The present invention relates to the manufacture of printed circuit boards having improved interlayer adhesion. More particularly, the present invention pertains to adhesiveless, flexible printed circuit boards having excellent thermal performance and useful for producing high-density circuits.
2. Description of the Related Art
Printed circuit boards are employed in a wide variety of applications. For example, they can be found inside radio and television sets, telephone systems, automobile dashboards and computers. They also play an important role in the operation of airborne avionics and guidance systems. Polyimide films are used in the production of circuit boards because of their excellent flex characteristics and good electrical properties. More particularly, it is common to attach a layer of a conductive metal foil to a surface of a polyimide film to provide a surface upon which a pattern of an electrical conductor can be provided. In such cases, it has been recognized in the art that any movement of the metal foil on the polymeric film could potentially impair the performance of the equipment incorporating the circuit board. To avoid this problem, it is necessary that the conductive metal layer be strongly adhered to the polymeric substrate to prevent any shifting of the metal layer on the film.
There have been various efforts in the art to improve the adhesion of metal foils to polymeric substrates in forming printed circuit boards while maintaining good thermal resistance and low cost of manufacture. U.S. Pat. No. 4,382,101 offers one proposed solution to this problem wherein a substrate is etched with a plasma etchant and then a metal is vapor deposited onto the etched surface of the substrate. This process requiring the vapor deposition of a metal directly onto an etched surface is very expensive. U.S. Pat. No. 4,615,763 provides a method of improving adhesion of a photosensitive material to a substrate by selectively etching resinous portions of a substrate comprising a resinous material and an inorganic particulate material. U.S. Pat. No. 4,639,285 teaches a process wherein a metal foil is attached to a surface of a synthetic resin substrate via an intermediate silicone-based adhesive layer after treating the substrate surface with a low temperature plasma. The low temperature plasma utilized is an organo-silicon compound with an inorganic gas, such as oxygen. U.S. Pat. No. 4,755,424 provides a polyimide film produced from a polyimide containing a dispersed inorganic powder. Particles of the inorganic powder protrude from the film surface to roughen the film. The film surfaces are then treated with a corona discharge treatment to alter the surface chemistry of the film. U.S. Pat. No. 4,863,808 teaches a polyimide film coated with a vapor deposited chromium layer, a vapor deposited copper layer, and followed by electroplating with copper. U.S. Pat. No. 5,861,192 provides a wet chemistry method with mechanical and projection grinding to increase the adhesion of a polyimide film surface.
The present invention provides an improved solution over those of the prior art. A process for forming printed circuit boards is provided wherein a polymeric film is coated onto at least one surface of an etched polymeric substrate followed by laminating a metal foil onto the coated film. The substrate surface may be etched with either a chemical or plasma etchant, and may comprise either the same or a different material than the polymeric film. The result is a circuit board with a substrate that exhibits high thermal resistance and excellent electrical insulating properties.
The invention provides a process for forming a printed circuit board composite comprising:
a) etching at least one of two opposite surfaces of a planar polymeric substrate;
b) attaching a polymeric film onto one or both etched surfaces of the polymeric substrate; and
c) laminating and attaching a metal foil onto the polymeric film.
The invention further provides a process for forming a printed circuit board comprising:
a) etching at least one of two opposite surfaces of a planar polymeric substrate;
b) attaching a polymeric film onto one or both etched surfaces of the polymeric substrate;
c) laminating and attaching a metal foil onto the polymeric film;
d) depositing a photoresist onto the metal foil;
e) exposing and developing the photoresist, thereby revealing underlying portions of the metal foil; and
f) removing the revealed underlying portions of the metal foil.
It is also within the scope of the invention to form multilayered printed circuit boards or composites by incorporating additional polymeric films or metal foil layers. A description of these embodiments is included herein.
The invention provides a printed circuit board support having improved interlayer adhesion, enhanced thermal stability and excellent electrical insulating properties as compared to the prior art.
The first step in the process of the invention is to etch at least one surface of two opposite surfaces of a suitable substrate with an appropriate etchant, thereby forming a first etched surface. Typical substrates are those suitable to be processed into a printed circuit or other microelectronic device. Preferred substrates for the present invention are polymeric substrates and non-exclusively include materials comprising polyester, polyimide, liquid crystal polymers and polymers reinforced with materials such as fiberglass, aramid (Kevlar), aramid paper (Thermount), polybenzoxolate paper or combinations thereof. Of these a polyimide substrate is the most preferred. Also suitable are semiconductor materials such as gallium arsenide (GaAs), silicon and compositions containing silicon such as crystalline silicon, polysilicon, amorphous silicon, epitaxial silicon, and silicon dioxide (SiO2) and mixtures thereof. The preferred thickness of the substrate is of from about 5 xcexcm to about 200 xcexcm, more preferably from about 5 xcexcm to about 50 xcexcm.
Appropriate etchants are those which are capable of selectively removing portions of the substrate surface. Preferred etchants for the present invention non-exclusively include plasma etchants and concentrated aqueous etching solutions.
Preferred are aqueous alkaline solutions, non-exclusively include Group I or Group II hydroxides which include hydroxides of elements from Groups I or II of the periodic table, such as sodium hydroxide and potassium hydroxide. Ammonium hydroxide may also be used. The useful concentration of an aqueous etchant varies with the chemical composition of the substrate to be etched. Typically useful etchant concentrations range from about 5% to about 25% by weight of the etchant material, preferably from about 10% to about 20%. For example, one useful aqueous etchant is a potassium hydroxide solution having a concentration of from about 8% to about 12% of potassium hydroxide. Also suitable is a sodium hydroxide solution at a concentration of from about 8% to about 16% by weight of sodium hydroxide.
Any plasma etching technique which is suitable for etching polymer substrates may be used. This plasma etchant is a highly charged gas that bombards the film surface with positive and negative charged species causing impurities on the surface to degrade as well as ablating the film surface. These include halogen containing plasma etching materials and oxygen containing plasma etching materials. The preferred plasma etchant comprises a gaseous mixture of oxygen (O2) and tetrafluoromethane (CF4). Preferably the plasma etchant comprises at a mixture of oxygen plasma and tetrafluoromethane plasma comprising least about 3% of tetrafluoromethane, more preferably it comprises from about 3% to about 20% and still more preferably from about 7% to about 20% of tetrafluoromethane with the balance being oxygen. This minimum quantity of tetrafluoromethane is important to prevent any over etching of the substrate.
The etching step of the process of the present invention is accomplished by contacting the polymeric film with the aqueous base etchant or plasma etchant.
Etching is conducted by contacting the areas of the substrate to be etched with the etchant material. Plasma etching is conducted under conditions sufficient to remove at least about 0.45 xcexcm from at least one surface of the substrate. Such procedures are well known in the art. In another embodiment of the invention, both surfaces of the substrate are etched, allowing additional layers to be added to the printed circuit board support of the invention having superior adhesion to the substrate.
When using an aqueous base etchant, the duration of the etching step is also determined based on the chemical composition of the substrate and is generally from about 10 seconds to about 4 minutes in length. For example, when using a KOH etchant, the etching time for a polyimide substrate is from about 20 seconds to about 3 minutes. Preferably the etching solution is maintained at a temperature of from about 40xc2x0 C. to about 65xc2x0 C. It has been found that neutralizing the surface with a dilute mineral acid to form a soluble salt and subsequently rinsing clean the surface with deionized water is desirable. Further, by altering the film residence time, the etch rate can be altered.
When etching is done by plasma etching, it may be performed in a plasma etching chamber as is well known in the art.
The next step is to apply a polymeric film onto one or both etched surfaces of the polymeric substrate. The polymeric film is preferably deposited onto the film as a liquid by coating, evaporation or vapor deposition to allow for control and uniformity of the polymer thickness. Preferred polymeric materials include polyimides, polyesters, polyester containing co-polymers, polyarylene ethers, liquid crystal polymers, polyphenylene ethers, amines, and combinations thereof.
Of these, polyimides are the most preferred. In another embodiment of the invention the polymeric film and the polymeric substrate comprise the same polymer.
Polyimides are preferred for the polymeric film because they have high electrical strengths, good insulating properties, a high softening point and are inert to many chemicals. Preferred are polyimides having a glass transition temperature (Tg) of from about 160xc2x0 C. to about 320xc2x0 C., with a glass transition temperature of from about 190xc2x0 C. to about 270xc2x0 C. are preferred. Preferably, the polymeric film will have a thickness of from about 2 xcexcm to about 100 xcexcm, more preferably from about 5 xcexcm to about 50 xcexcm.
The polymeric film may be applied to the polymeric substrate by coating a suitable solution onto the substrate and drying. The solution may be comprised of polymer precursors, a mixture of precursors and polymer or just polymer and an organic solvent. It is preferred that a single solvent be used in each solution. Useful solvents include acetone, methyl-ethyl ketone, N-methyl pyrrolidone, and mixtures thereof. The most preferred single solvent is N-methyl pyrrolidone. The polymer-solvent solution will typically have a viscosity ranging from about 5,000 to about 35,000 centipoise with a preferred viscosity in the range of 15,000 to 27,000 centipoise. The solution may comprise from about 10% by weight to about 60% by weight of polymer, more preferably from about 15% by weight to about 30% by weight of polymer with the remaining portion of the solution comprising one or more solvents. After application, the solvent is evaporated leaving a polymeric film on the substrate. Alternatively, a thin sheet of the polymer may be laminated under heat and pressure onto the substrate. In another embodiment, a molten mass of the polymer material may be extrusion coated onto the substrate.
The polymer film may also optionally incorporate a filler material. Preferred fillers non-exclusively include ceramics, boron nitride, silica, barium titanate, strontium titanate, barium strontium titanate, quartz, glass beads (micro-spheres), aluminum oxide, non-ceramic fillers and combinations thereof. If incorporated, a filler is preferably present in an amount of from about 5% to about 80% by weight of the film, more preferably from about 10% to about 50% by weight of the film.
Next, a metal foil is laminated to the substrate surface on which the polymer film has been formed. Lamination is preferably conducted by autoclave lamination, vacuum hydraulic pressing, non-vacuum hydraulic pressing or by hot roll lamination. Lamination may also be conducted using an ADARA(trademark) press which comprises heating the metal foil by an amount sufficient to soften the polymeric film by flowing an electric current through the foil and attaching the polymeric film to the substrate. When using a vacuum press, lamination is preferably conducted at a minimum of about 275xc2x0 C., for about 5-30 minutes. Preferably, the press is under a vacuum of at least 28 inches of mercury, and maintained at a pressure of about 150 psi.
Preferred metal foils for the printed circuit board support of the invention comprise copper, zinc, brass, chrome, nickel, aluminum, stainless steel, iron, gold, silver, titanium and combinations and alloys thereof. Most preferably, the metal foil comprises copper. Copper foils are preferably produced by electrodepositing copper from solution onto a rotating metal drum as is well known in the art. The metal foil preferably has a thickness of from about 3 xcexcm to about 200 xcexcm, more preferably from about 5 xcexcm to about 50 xcexcm. Alternatively, wrought copper foils may be used. However, the rolling process is effectively limited to producing foils no thinner than 18 microns.
The one or both sides of the metal foil may optionally be roughened, such as by micro-etching, by being electrolytically treated on the shiny side to form a roughened copper deposit, and or by being electrolytically treated on the matte side to include the deposition of micro-nodules of a metal or metal alloy on or in the surface. These nodules are preferably copper or a copper alloy, and increase adhesion to the polymer film. The surface microstructure of the foil may be measured by a profilometer, such as a Perthometer model M4P or S5P which is commercially available from Mahr Feinpruef Corporation of Cincinnati, Ohio. Topography measurements of the surface grain structure of peaks and valleys are made according to industry standard IPC-TM-650 Section 2.2.17 of the Institute for Interconnecting and Packaging Circuits of 2115 Sanders Road, Northbrook, Ill. 60062. The surface treatments are carried out to produce a surface structure having peaks and valleys which produce roughness parameters wherein the average roughness (Ra) ranges from about 1 to about 10 microns and the average peak to valley height (Rz) ranges from about 2 to about 10 microns.
The optional roughening of the shiny side of the foil is preferably carried out to produce a surface structure having peaks and valleys which produce with a roughness parameters wherein Ra ranges from about 1 to about 4 microns, preferably from about 2 to about 4 microns, and most preferably from about 3 to about 4 microns. The Rz value ranges from about 2 to about 4.5 microns, preferably from about 2.5 to about 4.5 microns, and more preferably from about 3 to about 4.5 microns.
The optional roughening of the matte side of the foil are preferably carried out to produce a surface structure having peaks and valleys which produce a roughness parameters wherein Ra ranges from about 4 to about 10 microns, preferably from about 4.5 to about 8 microns, and most preferably from about 5 to about 7.5 microns. The Rz value ranges from about 4 to about 10 microns, preferably from about 4 to about 9 microns, and more preferably from about 4 to about 7.5 microns.
An optional copper deposit on the shiny side of the foil will preferably produce a copper deposit of about 2 to about 4.5 xcexcm thick to produce an average Ra value of 2 xcexcm or greater. An optional nodule deposit on the matte side preferably will have an Ra value as made of about 4 to about 7.5 xcexcm. The micro-nodules of metal or alloy will have a size of about 0.5 xcexcm. Other metals may be deposited as micro nodules if desired, for example, zinc, indium, tin, cobalt, brass, bronze and the like. This process is more thoroughly described in U.S. Pat. No. 5,679,230.
In the preferred embodiment of the invention, the shiny surface preferably has a minimum peel strength of about 0.7 kg/linear cm., preferably from about 0.7 kg/linear cm, to about 1.6 kg/linear cm, more preferably from about 0.9 kg/linear cm to about 1.6 kg/linear cm. The matte surface preferably has a minimum peel strength of about 0.9 kg/linear cm and preferably from about 0.9 kg/linear cm to about 2 kg/linear cm, more preferably from about 1.1 kg/linear cm to about 2 kg/linear cm. Peel strength is measured according to industry standard IPC-TM-650 Section 2.4.8 Revision C.
In another embodiment of the invention, prior to lamination of the metal foil onto the polymeric film, a thin metal layer may optionally be electrolytically deposited onto either side of the metal foil. After lamination of the metal foil to the polymeric film, a thin metal layer may optionally be deposited onto the foil surface opposite the polymeric film by coating, sputtering, evaporation or by lamination onto the foil layer. Preferably the optional thin metal layer is a thin film and comprises a material selected such as nickel, tin, palladium platinum, chromium, titanium, molybdenum or alloys thereof. Most preferably the thin metal layer comprises nickel or tin. The thin metal layer preferably has a thickness of from about 0.01 xcexcm to about 10 xcexcm, more preferably from about 0.2 xcexcm to about 3 xcexcm.
The resulting laminate will have a peel strength that varies widely based on the thickness of the polymeric layers and the amount of substrate surface removal. For example, in order to obtain a laminate having an adequate peel strength of at least 4 lbs/inch, it is necessary to remove at least 0.45 xcexcm from the substrate surface.
After the metal foil has been laminated onto the coated substrate, the next step is to selectively etch away portions of the metal foil or metal foil and optional thin metal layer, forming an etched pattern of circuit lines and spaces in the foil, or in the foil and optional thin metal layer. This etched pattern is formed by well known photolithographic techniques using a photoresist composition. First, a photoresist is deposited onto the metal foil or optional thin metal layer. The photoresist composition may be positive working or negative working and is generally commercially available. Suitable positive working photoresists are well known in the art and may comprise an O-quinone diazide radiation sensitizer. The O-quinone diazide sensitizers include the o-quinone-4-or-5-sulfonyl-diazides disclosed in U.S. Pat. Nos. 2,797,213; 3,106,465; 3,148.983; 3,130,047; 3,201,329; 3,785,825; and 3,802,885. When O-quinone diazides are used preferred binding resins include a water insoluble, aqueous alkaline soluble or swellable binding resin, which is preferably a novolak. Suitable positive photodielectric resins may be obtained commercially, for example, under the trade name of A7-P4620 from Clariant Corporation of Somerville, N.J. as well as Shipley I-line photoresist. Negative photoresists are also widely commercially available.
The photoresist is then imagewise exposed to actinic radiation such as light in the visible, ultraviolet or infrared regions of the spectrum through a mask, or scanned by an electron beam, ion or neutron beam or X-ray radiation. Actinic radiation may be in the form of incoherent light or coherent light, for example, light from a laser. The photoresist is then imagewise developed using a suitable solvent, such as an aqueous alkaline solution, thereby revealing underlying portions of the metal foil or optional thin metal layer.
Subsequently, the revealed underlying portions of the metal foil or metal foil and optional thin metal layer are removed through well known etching techniques, such as acid or alkaline etching, while not removing the portions underlying the remaining photoresist. Suitable etchants non-exclusively include acidic solutions, such as cupric chloride (preferable for etching of nickel) or nitric acid (preferable for etching of tin). Also preferred are ferric chloride or sulfuric peroxide (hydrogen peroxide with sulfuric acid). Suitable etchants also non-exclusively include alkaline solutions, such as ammonium chloride/ammonium hydroxide.
If the optional thin metal layer is included, this step will reveal the portions of the metal foil underlying the etched off portions of the thin metal layer. This patterned thin metal layer is then useful as a high quality etch mask for etching the metal foil. After the thin metal layer is etched, the next step is to remove the revealed underlying portions of the metal foil by etching while not removing the portions of the metal foil underlying the non-removed portions of the optional thin metal layer, revealing portions of the polymeric film underlying the etched metal foil.
If the optional thin metal layer is not included, the metal foil is directly etched to revealing the portions of the polymeric film underlying the etched off portions of the metal foil. The laminate may then be rinsed and dried. The result is a printed circuit board having excellent resolution and uniformity, good thermal resistance and excellent interlayer adhesion.
After the circuit lines and spaces are etched through the metal foil, the remaining photoresist can optionally be removed either by stripping with a suitable solvent or by ashing by well known ashing techniques. The photoresist may also be removed after etching the optional thin metal layer, but prior to etching the metal foil.
In another preferred embodiment of the invention, the above processes may be repeated on an opposite side of the substrate. In this embodiment, two opposite surfaces of the substrate are etched by the techniques described above, and a layer of a polymeric film material may be coated or laminated onto each etched surface, followed by lamination of a metal foil layer onto each polymeric film. Each metal foil layer may then be patterned and etched by the techniques described above, including by using an optional thin metal layer.
The following non-limiting examples serve to illustrate the invention: